In known magnetic resonance devices, a magnet serves to generate a main or basic magnetic field with a strength of several T. The magnet itself consists of superconductive materials, which is why a corresponding cooling device is provided in order to be able to cool the magnet sufficiently vigorously and to maintain it in a superconductive state. The magnet together with its assigned cooling device is arranged in a vacuum housing. Normally, in known magnetic resonance devices with a central bore, a cylindrical vacuum housing, and therefore also a cylindrical magnet are provided. A gradient coil, which via corresponding alternating fields allows local coding of the magnetic resonance signals recorded, follows the vacuum housing radially inwardly. The basic structure of a magnetic resonance device is sufficiently known and does not have to be described in detail.
The gradient coil, which normally consists of a plurality of single coils, generates in addition to the actual gradient fields stray fields which lead to eddy currents in adjacent conductive structures. Lying closest to the gradient coil is, as stated, the vacuum housing or its inner wall. The vacuum housing itself is in known magnetic resonance devices formed completely from high-grade steel, therefore from a non-magnetic, but electrically conductive material. These induced eddy currents in particular in the vacuum housing have a detrimental impact on the characteristics of the system. One problem of these induced eddy currents is the noise or sound development which they give rise to. The vacuum housing, primarily its inner wall and optionally the end walls, is caused to vibrate due to the Lorentz forces generated by the changing eddy currents resulting from the high-frequency control of the gradient coils. This area, i.e. that of the vacuum housing, constitutes one of the chief sources of noise in normal magnetic resonance devices. A further problem is the negative affect on the imaging by fields induced in return by these eddy currents, which for their part act so as to generate a magnetic field, said eddy-current-induced magnetic fields also extending into the area of the gradient coil and acting detrimentally there. Stray fields resulting from these induced eddy currents can, furthermore, where they penetrate the vacuum vessel as far as the cooling device, in which liquid helium normally circulates, lead to an increase in helium boil-off. The principal cause is in each case the induced eddy currents resulting from the gradient stray fields.
From DE 102 29 491 A1 a magnetic resonance tomography apparatus is known, having damping laminated sheets for reducing vibrations, so as to reduce the transmission of noise when the tomography appliance is operating. To this end, a damping layer is provided between inner and outer sheets respectively.
In the magnetic resonance tomography apparatus having noise suppression through damping of mechanical vibrations which is described in DE 101 47 745 A1, damping elements made of a material with an electrostrictive property are provided on an inside of a magnet housing for absorbing acoustic vibrations that are produced when a gradient coil system is switched.
The magnet assembly for magnetic resonance imaging, which is known from U.S. Pat. No. 6,157,276, has an inner wall which is composed of a non-conductive material. In addition, a vapor barrier is provided that is produced by wrapping a thin ribbon of non-magnetic metal around an inner layer of the electrically insulating material of the inner wall.
DE 44 14 371 A1 describes for a diagnostic magnetic resonance apparatus a high-frequency screen between a high-frequency antenna and a gradient coil system, having a layer arrangement comprising adjacently arranged track conductors which are separated from one another by electrically insulating slots.